NITRIFICATION AND NITROGEN MINERALIZATION IN A LOWLAND RAINFOREST SUCCESSION IN COSTA-RICA, CENTRAL-AMERICA

N availability is a critical component of productivity in successional lowland rain forests, and N losses from a given system may largely depend on rates of nitrification in soils of the system. Two hypotheses were tested in a study of a 6-point secondary rain forest sere in the coastal lowlands of Costa Rica: that nitrification and N mineralization change in a directed fashion in lowland rain forest successions, and that nitrification is regulated by ammonium availability at all points along the sere. Nitrate and mineral N production were measured in short-term laboratory incubations of soils from different stages of secondary succession corresponding to 0, 3, 8, 16, 31 and 60+ yr following disturbance. Nitrification increases through the first 4 successional stages and then declines somewhat before leveling off. In soils from all sites, most of the N mineralized was nitrified, and added NH4Cl strikingly stimulated net nitrate production. Added NaH2PO4, CaCO3 and CaSO4 did not stimulate net nitrate production or did not result in a greater proportion of nitrate than in controls. Nitrification and N mineralization may tend to increase through secondary rain forest succession; ammonium availability along the sere regulates rates of nitrification.